Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler (CAC) in the intake manifold, the sensor may be sensitive to the presence of water vapor and other diluents in the charge air flow. For example, during certain operating conditions such as increased charge airflow, condensate formed within the CAC may be released as water droplets into the charge airflow. Water droplets at the oxygen sensor may be incorrectly interpreted as EGR, thereby resulting in an overestimate of EGR. As a result, the aircharge required to deliver the desired torque may be overestimated and result in incorrect throttle control.
In one example, the issues described above may be addressed by a method for an engine comprising adjusting a position of a throttle based on a dilution threshold when a total aircharge dilution level is greater than the dilution threshold, the total aircharge dilution level based on an output of an intake oxygen sensor. In one example, an estimated EGR rate may be determined based on the total aircharge dilution level, assuming EGR is the major diluent in the aircharge.
As one example, a dilution threshold may be based on a saturation vapor pressure at a throttle inlet temperature. The dilution threshold may be further based on a target EGR rate when EGR is flowing (e.g., when an EGR valve is open). When the total aircharge dilution level (e.g., decrease in intake oxygen due to diluents in the aircharge) is less than the dilution threshold, the throttle may be adjusted based on the total aircharge dilution level. However, when the total aircharge dilution level is greater than the dilution threshold, water droplets may be in the aircharge and increasing the dilution level. As a result, the throttle may be adjusted based on the dilution threshold rather than the higher total aircharge dilution level. Spark timing may be adjusted based on the total aircharge dilution level even when the total aircharge dilution level is greater than the dilution threshold. In this way, the aircharge required to deliver the desired torque may not be overestimated, thereby resulting in the engine delivering the demanded torque.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.